Heat-treatment of steels

ABSTRACT

A process for treating wrought steel containing up to 0.10 percent carbon, 0.0-0.5 percent Mn, 0.0-0.05 percent P, o.0-0.05 percent S, 0.0-0.005 percent N2 and 0.0-0.1 percent A1, comprises heating said wrought steel to a temperature above the austenitization point but below the melting point of the steel and quenching said heated wrought steel at a selected cooling rate ranging from 100* C./sec. to 500* C./sec. to below transformation temperature range, said selected cooling rate providing in the treated wrought steel a tensile strength having an essentially consistent value for any selected carbon content in said wrought steel up to about 0.10 percent carbon, and said essentially consistent tensile strength value being independent of the carbon content in said wrought steel up to about 0.10 percent carbon.

United States Patent [54] HEAT-TREATMENT OF STEELS 5 Claims, 3 Drawing Figs.

[52] US. Cl 148/143, 148/12,148/12.4,148/36, 148/134 [51] Int. Cl C21d 1/00 [50] Field of Search 148/143,

Primary ExaminerRichard 0. Dean Attorney-Cushman, Darby & Cushman ABSTRACT: A process for treating wrought steel containing up to 0.10 percent carbon, 0.0-0.5 percent Mn, 0.0-0.05 percent P, o.00.05 percent S, 0.0-0.005 percent N and 0.0-0.1 percent A1, comprises heating said wrought steel to a temperature above the austenitization point but below the melting point of the steel and quenching said heated wrought steel at a selected cooling rate ranging from 100C./sec. to 500C./sec. to below transformation temperature range, said selected cooling rate providing in the treated wrought steel a tensile strength having an essentially consistent value for any selected carbon content in said wrought steel up to about 0.10 percent carbon, and said essentially consistent tensile strength value being independent of the carbon content in said wrought steel up to about 0.10 percent carbon.

E k ,5 6O I (I). K F E8 0 H c b' 40 a I e a 1 I l I 0 02 004 0-06 0-08 0-1 Carbon PATENTEBum 26 an SHEET 10F 3 HEAT-TREATMENT F STEELS Low carbon mild steel sheet, strip and blackplate is normally made by subjecting the steel ingots to a sequence of hot and cold rolling processes to produce the fiat rolled product of desired width, gauge and surface finish. After the cold rolling process, the product is normally annealed to remove the effects of the working process. The final mechanical properties of the material are more or less in practice controlled by the particular features of the terminal annealing process.

To produce a soft ductile material with high formability the material is subjected to a lengthy subcritical annealing batch process in which the steel is slowly heated to 650700 C. and held for a time at this temperature and then slowly cooled. The mechanical properties of the material are primarily dependent on the final structure of the steel: although ultimately determined by the final annealing process it is also influenced by chemical composition, the type of teeming process (degree of deoxidation of the melt), the structure developed during the hot rolling process and the amount of cold work. Everything else being equal, the higher the subcritical annealing temperature and the longer the soaking time (time at annealing temperature), the more the carbides become spheroidized and the coarser becomes the ferrite grain size due to grain growth. Depending on the time and temperature of this cycle, the properties of the final product normally can vary between 18 to 23 tons/inches tensile strength and the most ductile grades show a total elongation in a 2-inch gauge length of approximately to percent.

When the specifications recrystallization the properties of the finished product are not governed by overriding ductility requirements but call for an engineering material of increased strength the same material can be processed by a short-time, continuous, subcritical (less than 723 C.) which induces primary recrystallization of the cold worked ferrite and restores ductility to the material. The short time at the recrystallization temperature does not allow grain growth and there is insufficient diffusion time for the carbides to spheroidize to an appreciable degree. This continuous annealing cycle is of a short duration in contrast to the process or batch anneal.

In both these types of annealing treatment, rates of heating are governed primarily by heat transfer, design and economic considerations, Cooling rate is determined in the case of batch annealing by the heat transfer problems inherent to the geometry of a closely wound coil which can be considered as a series of adjacent laps of steel interspersed with more or less dead gas spaces, In the subcritical continuous strand annealing process, the cooling rate is dependent on metallurgical considerations in that it must be slow enough to ensure that the carbon in solution at the elevated temperature of recrystallization, precipitates on existing carbides during cooling. That is, when the annealed material is at room temperature, there is no significant amount of supersaturated dissolved carbon in the ferrite. This ensures that the properties of the annealed product are stable and quench ageing due to the precipitation of supersaturated carbon does not occur. (An alternative to this type of cooling cycle is to deliberately cool rapidly from the subcritical recrystallization temperature and by controlling the coiling temperature to which the steel is quenched, induce the precipitation of the supersaturated carbon so that the mechanical properties of the flat rolled product are stable when it is removed from the coiler).

The thin sheet or blackplate produced in the conventional continuous strand annealer, due to its fine grain size and relatively fine dispersion of the carbide is harder, stiffer and less ductile than the batch annealed product: its tensile strength being up to 27 tons/inches and the elongation in a 2-inch gauge length varying up to 16 percent.

For applications where ductility is not a major requirement, this stronger continuously annealed material has enabled the steelmaker to reduce thickness of tinplate and so reduce the cost to the packaging industry. In order that tinplate may complete with other newer packaging material even stronger strip will be required to enable further gauge reductions to be made.

There are various means of extending the stiffness and strength of these types of materials. One method is by the addition of alloying elements when the steel is in the molten stage. Examples of this are the addition of P or/and N. This technique has disadvantages such as the increased difiiculties of rolling the stronger material down to thin gauges; the increased segregation effects in the ingot and the increased cost of steelmaking. A second method is to produce the material in the conventional manner to a thicker gauge than that required and following the conventional reduction and annealing, a further cold reduction is imparted to the material to bring it to the final gauge. Up to 50 percent final cold reduction is used in the tinplate industry. However, the resultant increase in strength due to work hardening is accompanied by a severe decrease in ductility and a marked increase in directionality of the properties in the plane of the sheet.

Other heat treatment processes have been developed to produce enhanced mechanical properties in this type of material. A rapid subcritical annealing process may incorporate a rapid quench from the recrystallization temperature to an ageing temperature at which the approximate 0.02 percent supersaturated carbon in the ferrite precipitates in a strengthening form (quench ages). For example, a 0.06 percent total carbon steel after recrystallization, quenching and ageing at 50 C. acquires a tensile strength of approximately 50 tons/inches? However, this property is unstable being sharply reduced if the material is subsequently exposed to service conditions in which the temperature is higher than the original 50 C. ageing temperature, as for example, in the autoclaving of a tin can.

Another proposed heat treatment that enhances the tensile strength involves the heating of the alloy to intracritical temperatures (between the Ac and the Ac transformation points) followed by a drastic quench to produce a structure of pools of martensite in a ferritic matrix. But from the viewpoint of producing an engineering material of consistent properties, the difficulty of controlling the size and distribution of the martensite pools in the ferrite matrix (in the case the structural feature which controls the tensile properties), makes this process impractical, In practice, the normal variation in the size and distribution of the carbide phase in the original hot rolled material plus the normal limits of accuracy of the in tracritical temperature to which the material can be heated will produce wide variations in structure and hence properties.

In the thinner gauges a low carbon martensitic product can be obtained by fully austenitizing the material (e.g. above 900 C.) and drastically quenching it (at a cooling rate greater than 2,000 C./sec.). A relatively stable and strong material is obtained whose properties can be varied by a subsequent shorttime tempering treatment. The mechanical properties of this low carbon martensitic material in the as-quenched condition vary according to (a) chemical composition, (b) quenching rate, and (c) austenitic grain size. With an adequate quench rate and everything else being equal, the as-quenched tensile strength varies from approximately 60 to tons/inches according to the amount of minor constituents, the most important of these being the carbon. The ductility as indicated by the elongation in the uniaxial tensile test is rather limited and varies from 1 to 2 percent in a 2-inch gauge length.

Due to the inherent heterogeneity of a large steel ingot, a coil of the flat rolled product may have a carbon variation within it of from 0.05 to 0.10% C. This indicates that a martensitic material produced from a single ingot may have its tensile properties varying from 60 to 90 tons/inch. The rapid tempering of such materials in, for example, a strand annealing process to produce a more ductile material with the still enhanced strength of a tempered martensite would retain a wide variation in its mechanical properties.

An engineering material with a consistent enhanced strength intermediate between the continuously subcritically annealed and martensitic conditions (that is between 27 tons/inch and 60+tons/inch), without the inherent deficiencies of the cold worked material previously referred to (eg double reduced tinplate) and with considerable ductility is highly desirable.

it has now been found that a low carbon mild steel sheet, strip or blackplate of superior, consistent mechanical properties can be produced by austenitizing the steel followed by quenching at cooling rates between and including 100 C./sec. and 500 C./second The consistent mechanical properties of the products of this process are obtained regardless of the chemical composition of the steel (within the normal range occuring in low carbon mile steels, i.e. steels containing up to 0.10% C.), teeming procedure or prior thermal and mechanical history. The product has, for example, a tensile strength of 40 tons/inch combined with a l2 to 15 percent total elonga tion and minimum directionality.

The requisite cooling rates may be obtained by choosing the appropriate quench medium for a particular set of heat transfer conditions related to the thickness of the steel, boundary conditions associated with the with the surface of the steel, etc. Thus for instance, cooling rates of l500 C./sec. may be obtained with flat rolled products up to 0.1 inch in thickness by quenching into a mineral oil bath maintained at the correct temperature. For the same thickness alloy or different size and shaped products of the alloy, when heat transfer conditions may or may not be different, the requisite cooling rate may also be achieved by other quenching media and techniques, including slurries, gas jets, controlled concentrations of gases in liquid media, liquid sprays controlled by the design of the jets.

Also, the process of the present invention may be used as a batch of a continuous process and, for example, may be incorporated into the conventional type of continuous or semicontinuous annealing plant used for sheet, strip of blackplate, by the provision of higher annealing temperature than the normal subcritical ones and the incorporation of a quench or cooling assembly on the exit side of the furnace to achieve the specified requisite cooling rates.

The process may be applied to articles of similar thickness made either from steel sheet or strip or by a process in which the thickness of the metal is similar to that of steel sheet, e.g. tubing.

Thus the present invention in its most general sense provides a process for producing a wrought low carbon mild steel product having mechanical properties substantially independent of the presence of alloying elements or prior mechanical and/or thermal treatments, said process comprising the steps of heating the product to a temperature above the austenitization point but below the melting point of the steel and then quenching the heated product at a cooling rate from 100 C./sec. to 500 C./sec. to below transformation temperature range. Wrought products include not only sheet, strip and plate but also wire, tubing and like materials.

The austenitization point of the steel will in general be 870 C. at the minimum and represents the temperature at which austenitization commences. This temperature may vary slightly according to the quality and nature of other alloying elements present in the steep but is unlikely to exceed 900 C. Generally, it will be preferable to heat the steel up to a temperature substantially above the austenitization point, but still below the melting point, in order that the transition to the austenitic state may proceed more rapidly. Temperatures of around l,O00 C. are suitable for this purpose. The lower limit of the transformation range of the steel represents the temperature below which no further phase changes take place on cooling.

Suitable alloying elements such as are conventionally used in low carbon metal steels, and which can be present, in corresponding conventional quantities, in steel products to be treated by the present process include Mn, P, S, N, Al. One preferred group of low carbon mild steels for use in the present process is represented by those containing 0.0-0.570 Mn, 0.0-0.05% P, 0.0-0.05% S, 0.00.05% N ,0.0-0.1% Al.

Advantages of the process are that the steel for a moderately high-strength level has more ductility than similar strength strip produced by normal means used in the production of tinplate such as double reduction (work hardening by cold rolling) and alloying and also significantly less directionality than the product of the former process. A further important advantage is that the process provides a means for treating a product of any chemical composition normally produced within this class of steel and/or of any thermal or mechanical history, to give an engineering product of consistent strength and ductility.

The invention is illustrated by the following example:

EXAMPLE Specimens from a range of steels A to 1 (table 1) were austenitized and immediately quenched into a variety of quench media to simulate a continuous annealing cycle and to observe the effect of quench rate on mechanical properties.

At high quench rates, the strength level is sensitive to chemical composition and throughout the range of steels investigated there is considerable spread in the tensile results, as shown in FIG. 1 of the attached drawings. in the experiments illustrated by FIG. 1 the specimens were quenched from l,000 C. at a cooling rate of 1 1,000" C./sec.

At progressively lower cooling rates tensile strength decreases and the variation in tensile properties for the range of steels becomes less marked, as shown in FIG. 2 of the attached drawings. In the experiments illustrated in FIG. 2, the specimens were quenched from 1,000 C. at a cooling rate of 1,000 C.lsec.

At rates equivalent to those produced by quenching thin gauge material into mineral oils (l00-500 C./sec.) a consistent product is produced regardless of chemical composition, teeming procedure (rimming or killed), or prior thermal and mechanical history, as shown in FIG. 3 of the attached drawings. This product has an ultimate tensile strength of 40 tons/inches, combined with [2- l 5 percent elongation; a most useful combination of strength and ductility in a thin gauge material.

In the experiments illustrated in FIG. 3 the specimens were quenched from 1,000 C. at a cooling rate of 300 C./sec., using mineral oil at 20 C. as quenching medium.

in the drawings, each point represents a separate reading. Thus in the results as shown in FIG. 1, four readings where obtained for steels, D, E, three for steels C, G and two readings for steels A, B, F, H, and l. ln the results shown in FIG. 2, three readings were obtained for steels E, F and two readings for each of the remaining steels, ln the results shown in FIG. 3. two readings were obtained for each A to I.

Table l is as follows:

TABLE 1 Percent Percent Percent Percent Percent Percent C Mn P S N; Al

0.088 0.37 0.018 0.030 0.0036 0.046 0.44 0.023 0.027 0.0035 0.065 0.29 0.018 0.03l 0.0035 0.05l 0.32 0.02l 0.033 0.0035 0.045 0.45 0.022 0.029 0.0027 0.032 0.30 0.024 0.0l8 0.0027 0.053 0.36 0.0l8 0.02! 0003B 0.068 0.061 0.4) 0.024 0.0 l 8 0.0028 0.57 0.020 0.020 0.0027

A process according to the present invention involving austenitization followed by controlled quenching for providing stronger sheet is also envisaged for applications other than those of the tinplate industry. Laboratory tests have shown that cooling rates of up to 3,000 C./sec. can be achieved on material of 0.040 inch thickness by choosing the appropriate quench medium and quenching conditions. Little difficulty is therefore envisaged in producing material up to at least 0.1 inch thick, quenched in the critical range of quench rates specified, to give steel sheet of consistent strength and formability.

the present invention also includes the low carbon mild steel sheet, strip and blackplate and other products of wrought na ture of superior consistent mechanical properties. produced by the process of the present invention.

We claim:

1. A process for treating wrought steel containing up to 0.10% carbon, 0.0-0.5% Mn, 0.00.5% P, 0.00.05% S, 0.0-0.005% N and -01% Al, comprising heating said wrought steel to a temperature above the austenitization point but below the melting point of the steel and quenching said heated wrought steel at a selected cooling rate ranging from 100 C./sec. to 500 C./sec. to below transformation temperature range, said selected cooling rate providing in the treated wrought steel a tensile strength ranging intermediate 27-60 tons/inches, said tensile strength having an essentially consistent value for any selected carbon content in said wrought steel up to about 0.10% carbon, and said essentially consistent tensile strength value being independent of the carbon content in said wrought steel up to about 0. 10% carbon.

2. The process of claim 1 wherein said wrought steel contains 003-0. 10% carbon.

3. The process of claim 2 wherein said wrought steel contains 0.032-0.088% carbon.

4. The process of claim 1 wherein heated wrought steel is quenched in mineral oil.

5. The process of claim 1 wherein the wrought steel is heated to l,000 C. and quenched at a cooling rate of about 300 C./sec. 

2. The process of claim 1 wherein said wrought steel contains 0.03-0.10% carbon.
 3. The process of claim 2 wherein said wrought steel contains 0.032-0.088% carbon.
 4. The process of claim 1 wherein heated wrought steel is quenched in mineral oil.
 5. The process of claim 1 wherein the wrought steel is heated to 1,000* C. and quenched at a cooling rate of about 300* C./sec. 